It is known to remove hydrogen sulfide and carbon dioxide in particular, from a gaseous mixture. A physically acting absorbing medium is contacted in an absorption column with the pressurized gaseous mixture so that the acidic gas is absorbed at least partially along with some of a useful gas, and the thus loaded absorption agent is subsequently extracted from the initial absorption column and, to recover the useful gas, is fed to a recycle flash container in which the loaded absorption agent will be decompressed to form a recycled gas that contains the useful gas and the acidic gas. The recycled gas can then be compressed for further use.
As a useful gas, the synthetic or natural gas mixture can contain for instance hydrocarbon compounds, or as acidic gas, hydrogen sulfide, organic sulfur compounds and/or carbon dioxide.
From a process point view as well as to avoid ecological damage, at least most of the acidic gas must be removed before further using a gas mixture such as natural domestic gas. In practice, chemically acting absorption agents as well as physically acting absorption agents such as for example polypropylene carbonate, methanol, N-methyl-pyrrolidon, N-formylmorpholine, N-acetylmorpholine, and mixtures of these substances are used. Appropriate absorption agents are for instance known under the brand names Selexol®, Pursiol®, Genosorb®, and Morphysorb®.
A certain portion of the useful gas is typically taken is up by the absorption agent when a gaseous mixture is cleaned in an absorption column using a physically acting absorption agent, which creates a problem. To be able to recover the useful gas U.S. Pat. No. 7,604,684 describes feeding the absorption agent after washing the gaseous mixture in an absorption column to a recycle flash container where the loaded absorption agent is decompressed. The decompression creates a recycle gas that contains useful and acidic gas components. The recycle gas formed in the recycle flash container is compressed for further use and, because of its high acid content, is fed to the gaseous mixture before it is cleaned using the physically acting absorption agent. The absorption agent, which was taken out of the recycle flash container, will be regenerated and fed again to the absorption column in a cycle.
Furthermore, it is known from practice to feed the recycled gas formed in the recycle flash container to an additional, smaller absorption column where the recycled gas will be separated at least partially from the acidic gas by means of a fresh, regenerated absorption agent. It is disadvantageous that an additional, regenerated absorption agent will be required thus increasing the total required quantity of absorption agent that is fed into a cycle. Accordingly, the entire cycle must be designed to allow large quantities of absorption agent to circulate so that the investment and operating costs are relatively high.